This invention relates to interference suppression apparatus and more particularly to an interference suppression apparatus suitable for use in satellite or other communication systems employing N-phase PSK (Phase Shift Keying) modulation.
Interference in a communication system may be defined as any unwanted signal or noise that interferes with reception or transmission of a desired signal. In communication systems employing directional antennas, such as satellite communication systems interference can be caused by the reception of cochannel signals through the side lobes of either the receiving satellite or ground station antenna.
Various processing schemes are known in the prior art for the reduction of interference. Techniques such as described by B. Saltzberg in U.S. Pat. No. 3,177,489 require an auxiliary directional canceling antenna to be aimed at each interfering source. In addition, the desired signal received by the auxiliary antenna must be weaker than the interference received by the auxiliary antenna. Such technique suffers the disadvantage that each canceling antenna must be aimed so that its received main lobe corresponds in direction to signals interfering with the desired signal of the main antenna and its received side lobes correspond in direction to the desired signal of the main antenna.
U.S. Pat. No. 3,435,453 issued to D. D. Howard describes the use of a canceling subarray in a phased array antenna system. The technique is basically that of replacing the auxiliary directional antenna as above described by a broadbeam phased array, and employing a side lobe canceler such as described in U.S. Pat. No. 3,202,990 issued to P. W. Howells. Such a canceler suffers the disadvantage, however, of being unable to avoid partial cancellation of the desired signal when desired signal energy enters the subarray with similar strength cochannel signals and noise as contemplated in communication system applications.
Another technique for interference suppression known in the prior art requires that the desired signal be modulated with a known wideband pseudorandom code. Such techniques involve the sacrifice of bandwidth and the use of additional spectrum space because the pseudorandom code used is usually many times the bandwidth of the transmitted signal.